I understand that we have to use the past tense after certain phrases like 'It is about time' and 'It is high time', e.g. It's about time I went home. It's high time I studied.

I thought the past tense is used above because of the subjunctive mood. But after second thought, it may not be correct, coz the past subjunctive is used for wishes, false or improbable conditions, while there's no such meaning in the 2 examples.

So, could you please tell me why the past form is used after the 'It's high time / It's about time'?